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Old 12-04-2019, 09:02 AM
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The_bluester (Paul)
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Non reducing correctors for SCT

I have been doing some googling but I have really not had any joy so far. Is anyone aware of non reducing corrector/flatteners for SCT scopes? Specifically the non Edge HD C925.

I am looking to work at my scopes native focal length for imaging smaller objects like many galaxies and PN's, but curvature and coma is visible even on a smaller sensor (ASI294)

I was using the Celestron 0.63 reducer spaced to give about 0.7 reduction but I am looking for the tighter image scale, it also exhibited internal reflections that I found irritating, they looked to be on glass to air transitions to probably a quality issue that is not readily fixable by tricks like blackening edges and threads better.

The budget will not stretch to replacing it with something like an Edge HD version for the flatter field, at least not for a long while.
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Old 12-04-2019, 11:21 AM
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sil (Steve)
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I don't think its really possible to do. Its why the edgehd versions are there. its to do with distortions throughout the optical train. Some optics use special elements which are very expensive but result in less distortion at the part of the train. look at a good cross section diagram of a camera lens (which is basically the same as an OTA. functionally the entire point is to take the light coming in at the front (full spectra "white" light) which is mostly travelling parallel paths and has to by the end of the optical train, bend it, magnify it etc and let it exit the end in parallel paths of white light from the end into a receiver: a piece of photographic film, a ccd/cmos digital camera chip, an eyeball.

in between the entrance and the exit the light is controlled to achieve a desired result, often magnification so that things in the sky that look small become large enough for us to see and record safely. first steps were basically just a magnifying glass set up and while magnifying glasses work well for watching ants we can still see distortions at the edge and best clarity in the centre. so work on shaping the glass and trying different types of glass gave us better magnifiers with more clarity across the entire element. The light of course is refracted as it travels through. this results in the white light being split into a complete rainbow spectra as each wavelength bends a different amount to all the rest so it splits and spreads out. typically the more you bend the light the wider it spreads.

I assume this is all obvious knowledge to you, so i apologise.

Now you have a lens element that takes the light coming in from a comnined parallel white input to a spread rainbow focused in on a smaller are for magnification. but its all trippy and psychedelic baby. so you attach a new glass element to the first to bend the light back outward to white parallel hopefully. this requires the two elements to not have an airgap so the surfaces much be precisely matched to each other. You can now put in an air gap and use more elements to magnify and straighten again and so on until the ouput is what you want. any tiny error in comstruction and imperfection in the elements effect how accurate they perform their tasks and when it comes to mass production each part has to be made and tested to with a certain margin of error so the then entire assembly performs within advertised range. So every camera lens and OTA of the same make and model will perform differently to each other. By random chance the one YOU buy might have all parts that are mathematically perfect but odd are it won't be. and how well quality control and testing is done at manufacture determines how well performing and consisten a model is. In the world of photography there are specific lenses within a range of serial numbers that are legendary for performance because the factory was more meticulous at the time but later change to cost cutting methods.

So In answer to your specific question at last all reducers/correctors are a compromise solution that works well across a range of possible optical characteristics in OTAs. By reducing you are taking whats there and shrinking to a smaller area, so it appears sharper with more detail and the larger obvious distortions are not removed, they are reduced so everything looks better though its the same, only smaller.

I think to correct at 0.99% I assume you have to start by measuring the distortion characteristics of the OTA itself and create an element specifically aligned to correct THAT OTA. I guess an average element could be made but I suspect it would create a blotchy correction field of sorts where parts of the field may be improved but parts may be worsened too in balance. Probably problematic to build. Best solution is what I've always known in photography: pay the extra to buy the quality glass. you got no way to upgrade a cheaper one to compete and are likely to buy the expensive anyone down the track, so save on fixes and cost of lesser optics in the first place, when you know what you want and need just go for the best from day one. not always practical of course and we are an impatient species.

I thought through something similar long ago, but the more you bend light and the more glass you pass it through the more energy is lost so you end up with more distortion and less contrast in the end. I thought using a microscope for an eyepiece would give amazing magnification. But the same still applies no matter how I thought on it. unless we can control the photons in a non contact way like magnetic field I think we are about as close to perfect for optics technology already.

I bet Merlin will jump in with a basic optics equation and reams of citations to prove it that will tell us in easy numbers why we have what we have and cant get better

cheers
s
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Old 12-04-2019, 11:41 AM
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The_bluester (Paul)
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I canít really complain too much. I am using an OTA photographically which was not built specifically for that purpose and that I have used as a visual instrument for a good long time first.

The issue is that the Edge HD tubes show that it IS possible to correct them nicely. Unless of course they are really an F15 or similar OTA with inbuilt reducer/corrector and not just correcting them at F10 for a flat field wothout reduction.

Even then I suppose they would be unlikely to want to produce an equivalent flattener for the standard scopes for a couple of hundred dollars when they can sell me an Edge HD instead for probably a lot better margin.
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Old 12-04-2019, 04:55 PM
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Paul, have a look for an alternative reducer/flattener. I think Starizona and/or Optec do one that works well with the original scopes.

In practice, you'll get better results with the reduction...unless you frequently get exceptional seeing, then it's unlikely you'd ever see a difference in detail between f/10 and f/6.3. And you'll get the reduced exposure times as a bonus...which you could use to gain extra images, and drizzle

Most of the time, we are really seeing-limited IMO
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Old 12-04-2019, 06:48 PM
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The_bluester (Paul)
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I might have to look at that, the Celestron reducer is fine for visual use and I used it a lot for that, but for AP the coatings are just not up to the task, big reflections and light loops from any bright stars in or near the FOV.

I have seen images by other people showing the same thing using that reducer so I assume it is endemic to them.

Last edited by The_bluester; 12-04-2019 at 07:47 PM.
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